INFLUENCE OF THE SAMPLE INTRODUCTION SYSTEM ON ACID EFFECTS IN INDUCTIVELY-COUPLED PLASMA-ATOMIC EMISSION-SPECTROMETRY

Influence of the aerosol formation and transport on acid effects has been studied in inductively coupled plasma atomic emission spectrometry in order to elucidate the decrease in the analyte intensity. To separate the role of the ionization and excitation conditions from that of the sample introduction, operating conditions have been set-up to obtain a constant ratio of the ionic line to atomic line intensity of magnesium. A 27 MHz crystal-controlled generator and a 40 MHz tuned-line generator were used to verify the possible influence of the generator type; Once the ionization and excitation conditions were kept constant, the influence of the sample introduction system could be assessed by using the Mg I 285 nm line. Perchloric acid up to a concentration of 50% (v/v) has been used as the test acid. The CII 725.662 nm line was also used to assess the amount of chlorine reaching the plasma. Several pneumatic nebulizers were used: Meinhard concentric nebulizers, a Perkin Elmer cross-flow nebulizer, and a Perkin Elmer cone-spray nebulizer. Depressive effects resulting from the perchloric acid depended on the nebulizer type, the carrier gas flow rate and the spray chamber design. Droplet size of the primary and tertiary aerosol were measured. At perchloric acid concentrations higher than 1% (v/v), results suggested that the depressive effect on the Mg line intensity cannot only be explained by a change in the aerosol density but also by a variation in the analyte concentration in the fraction of aerosol that reached the plasma. A careful selection of the sample introduction system components and parameters could minimize the acid effects. Results were compared with those obtained with an ultrasonic nebulizer associated with a desolvation system. Acid effects were more important for the ultrasonic nebulizer than for the pneumatic nebulizers and were explained by a reduction in the production of aerosol.